#include <iostream>
#include <cmath>
#include <stdio.h>
#define pi 3.1415926535

using namespace std;

double get_pi(int n){
    double a = 2 * pi / n;
    double high = sin(a);
    double lt_tr = high / 2;
    double s = lt_tr * n ;
    return s;
}

int main() {
    long long n;
    cin >> n;
    printf("%.11f\n",get_pi(n));
    return 0;
}